Rotational molding is generally used to produce a variety of hollow articles of manufacture which are suitable for a variety of applications such as outdoor sports equipment, storage containers and refuse containers. To rotationally mold a part having a desired configuration, a suitable molding resin is introduced into the interior of the rotational mold, which is then closed and rotated about one or two axes inside a heated oven to allow the charged resin to fuse together, forming a hollow article inside the mold. The mold is then removed from the oven to cool. Once cooled the resulting article of manufacture is removed from the mold.
As for specific materials employed as resins in rotational molding, it was indicated in an article appearing in Modern Plastics (mid-November 1995) at page D171:
Close to 80% of current rotational molding applications use low-, linear-low-, and high-density polyethylenes as well as crosslinked grades of polyethylene. Ethylene vinyl acetate and adhesive-type polyethylenes are also used in specialized applications. Other resins used are polyvinyl chloride, nylon, polycarbonate, polyesters and polypropylene.
A large variety of commercial rotational molding machines are available in the market to carry out the steps of a rotational molding operation.
In the rotational molding of articles of manufacture, there is often employed a foamable resin, which serves to form a foam that fills the otherwise hollow interior of the molded article. Such foamable resin further serves to reduce part weight, provide a higher insulation value and increased stiffness of the resulting article of manufacture.
Commercial applications are best satisfied by foam-filled plastic articles which exhibit, in addition to sufficient impact strength, a distinct, non-porous outer skin layer. It has long been recognized that articles having a solid skin and a foamed inner layer can be made by rotational molding processes, but to date these have enjoyed little or no commercial success, primarily owing to the fact that the outer skin layer produced often fails to be sufficiently smooth, thick and uniform to meet aesthetic or performance characteristics for different applications.
In the process disclosed in an early patent describing the production of a foamed composite article exhibiting a skin layer, U.S. Pat. No. 3,814,778 (Hosoda, et al.), a mixture is placed in a rotational mold, which comprises two components: (i) a powdered thermoplastic resin, and (ii) a partially crosslinked or crosslinkable granular thermoplastic resin. It is stated in Hosoda that the powdered plastic material tends to adhere to the entire inner surface of the mold to form the surface layer, then the granular plastic, in its turn, adheres to the surface layer.
Similarly, according to Canadian Patent No. 983,226 granted to Du Pont of Canada Limited in 1976, a one-step rotational molding process is used to arrive at articles having a solid skin and a foamed inner layer by using as the molding powder a mixture of "a powdered non-foamable ethylene polymer" and, in pellet form, "a foamable ethylene polymer".
A number of more recent patents provide specific variations on the known technique of combining a foamable resin and a second polymer of smaller particulate size or lower melting point than the first, so that the second polymer ultimately forms a skin layer of the foamed final article, for example, U.S. Pat. Nos. 5,532,282 and 5,366,675 (Needham).